A picture is needed. Don't understand the 1-1/4 line. You never reduce a drain line size in the direction of flow. Kitchen sink drain should be 1-1/2 unless there is a disposal then it should be 2". The laundry waste should be 2". All fixture should be trapped and vented. The only place that a 3" drain is needed on a clothes washer in in a chimerical laundry. And in that case it serves more then one machine.

John